1. Instructions for Locating an Earthquake Epicenter

2. Earthquake Waves
By determining the time of arrival for both the P and S waves at a location we can determine how far away the Earthquakes epicenter is from the seismograph.
This does NOT tell us what direction the waves are coming from.

3. Locating the Epicenter of an Earthquake Part I: Finding the distance to the epicenter
Step 1:
Determine the difference in arrival time for your P-wave, and your S-wave.
S-wave
03:27:15
P-wave
03:21:15

4. Calculation – difference in arrival time
03:27:15
-03:21:15
00:06:00 minutes

5. Locating the Epicenter of an Earthquake Part I: Finding the distance to the epicenter
Step 2:
Take out your ESRT. Open to the Travel Time Graph on page 11.
Step 3:
Use the vertical scale (time) to mark off the difference in arrival time on a scrap sheet of paper.
00:06:00 minutes

6. Locating the Epicenter of an Earthquake Part I: Finding the distance to the epicenter
Step 4:
Make sure to keep your scrap paper vertical! Slide it along the curves until it lines up on each of the curves.
Be very accurate!!!!!

7. Locating the Epicenter of an Earthquake Part I: Finding the distance to the epicenter
Step 5:
Read off the distance from the horizontal axis that corresponds to this spot. This is the distance between the epicenter and your seismograph location.
This distance is the radius of a circle around the seismograph.

8. Locating the Epicenter of an Earthquake Part I: Finding the distance to the epicenter
Step 6:
Repeat steps 1-5 for at least 2 other seismograph locations.
It is necessary to have at least 3 stations, if you do not you can not be sure of the exact location of the epicenter.
Common Regents Question

9. Locating the Epicenter of an Earthquake Part II Locating the Epicenter
Step 1:
Find the location of your first seismographic station on the map.
Step 2:
Use a compass or string to create a circle with its center at your seismograph location, and a radius equal to the distance you found.

10. Locating the Epicenter of an Earthquake Part II Locating the Epicenter
Step 3:
Repeat this procedure around two other seismographic stations.
Where the three circles all intersect (cross) is where your epicenter is located.
If the circles do not all intersect, but form a small triangle, the epicenter is the center of the triangle.

11. Locating the Epicenter
Epicenter Location

12. Figure 2.13

13. Locating the Epicenter of an Earthquake Part III: Origin time of the Earthquake
Step 1:
From the distances determined in Part I, determine how long it would take a P-wave to travel that distance.

14. Origin Time of Earthquake
P-wave Travel Time = 00:07:40
Step 2:
Find the distance on the horizontal axis.
Go up to the point where you hit the P-wave travel line.
Go over to the vertical axis and read off the travel time for the P-wave.
Distance = 4,400 km

15. Origin Time Story
We have the P-wave arrival time at the seismograph, and the time that it took to get there, so we need to find the time the wave left the focus of the earthquake (origin time of the seismic waves).
You went to a party, you arrived at 06:30:00, it took 00:12:15 to get there, what time did you leave your house?
Calculation:
06:30:00 – arrival time
-00:12:15 – travel time
06:17:45 – time of departure
Check your work:
06:17:45 – departure
+00:12:15 – travel time
06:30:00 – arrival time

16. Origin Time of Earthquake
Step 3:
Taking the time found in step 1, subtract this from the arrival of the P wave and that is the original time of the earthquake.
Calculation
03:21:15 - P-wave Arrival Time
-00:07:40 - Travel Time for 4,400km
03:13:35 - Origin Time (when the earthquake occurred)